Abstract

Background:

Breast cancer (BC) is a complex disease, consisting of molecular subtypes with different prognosis and possibly different etiology. We and others have previously reported that hormonal factors such as hormone therapy and pregnancies predominantly affect luminal-like BC, but it is unclear whether alcohol and physical activity are associated only with certain subtypes.

Methods:

We conducted a case-control study nested within a cohort of 457,036 women who participated in the Norwegian Breast Cancer Screening Program (NBCSP) in 2006-2012, and who completed a questionnaire at baseline screening. In all, 5,554 BC cases with information on risk factors and hormone receptor status (i.e. estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor 2 (HER2) occurred during the follow-up. For each case, 5 controls were randomly selected from among participants who did not develop BC, frequency-matched to the cases on year of birth and year of screening. The following surrogate definitions of BC subtypes were used: ER+PR+HER2- (“luminal A-like”), ER+PR-HER2- (“luminal B-like, HER2 negative”), ER+HER2+ (“luminal B-like, HER2 positive”), ER-PR-HER2+ (“HER2 positive”) and ER-PR-HER2- (“triple negative”). We used multinomial logistic regression to estimate odds ratios (ORs), with 95% confidence intervals (CIs), adjusted for age, body mass index (BMI), education, age at menarche, number of pregnancies and menopausal status.

Alcohol intake was positively associated, and physical activity negatively associated, with BC overall. Although these associations were statistically significant only for luminal A-like BC, their direction and magnitude were rather similar across several subtypes.